The invention relates generally to a scintillation camera, and more particularly to an apparatus for measuring the non-uniform attenuation of radiation caused by a patient in the scintillation camera.
Scintillation cameras are well known in the art of nuclear medicine, and are used for medical diagnostics. A patient ingests, or inhales or is injected with a small quantity of a radioactive isotope. The radioactive isotope emits radiations that are detected by a scintillation medium in the scintillation camera.
The scintillation medium is commonly a sodium iodide crystal, BGO or other. The scintillation medium emits a small flash or scintillation of light, in response to stimulating radiation, such as from a patient. The intensity of the scintillation of light is proportional to the energy of the stimulating radiation, such as a gamma ray. Note that the relationship between the intensity of the scintillation of light and the gamma ray is not linear.
A conventional scintillation camera such as a gamma camera includes a detector which convert into electrical signals gamma rays emitted from a patient after radioisotope has been administered to the patient. The detector includes a scintillator and an array of photomultiplier tubes. The gamma rays are directed to the scintillator which absorbs the radiation and produces, in response, a very small flash of light. The array of photodetectors, which are placed in optical communication with the scintillation crystal, convert these flashes into electrical signals which are subsequently processed. The signal processing enables the camera to produce an image of the distribution of the radioisotope within the patient.
Gamma radiation is emitted in all directions and it is necessary to collimate the radiation before the radiation impinges on the scintillation crystal. This is accomplished by a collimator which is a sheet of absorbing material, usually lead, perforated by relatively narrow channels. The collimator is detachably secured to the detector heads allowing the collimator to be changed to enable the detector head to be used with the different energies of isotope to suit particular characteristics of the patient study. The collimator may vary considerably in weight to match the isotope or study type.
Gamma rays emitted by a radioactive source, depending on its location, pass through different thicknesses and often different types of underlying or overlying tissue and therefore are attenuated by different amounts. As a result, a uniform distribution of radioactivity produces different counts at different locations of the organ, not a desirable feature in any imaging.
In conducting cardiac studies, there are usually areas of reduced radioactive readings from the patient due to self-attenuation by the body. This self-attenuation or self-absorption occurs because chest muscles tend to absorb radiation rather than emit it. Often, up to half to three quarters of the radioactivity is lost by self-absorption. In imaging the heart, areas of reduced activity in the heart muscle are seen, caused by the self-attenuation. This results in images that are inaccurate. Since the human body is generally of a non uniform shape and the heart is not centralized within the body, self-absorption must be measured. Therefore, readings are generally taken to measure or calculate how much radiation the patient absorbs in each view. These readings are then used to correct the readings from the patient activity to produce accurate images.
Known methods of measuring self absorption exist. However these methods are not reproducible in all cases. In many cases, these methods produce worse diagnostic results than without correction.
One method uses a radioactive source beamed through the patient and measures the absorption. Since the same amount of radiation will be absorbed each time, correction can be made to the created images. The problem with this method is that usually, the same detector head is used to detect the radiation from not only the patient but from the external source used to measure the attenuation. This practice reduces sensitivity and does not provide very accurate results.
Other methods require the use of an isotope of a high energy which does not provide a good measure of absorption since gamma rays emitted from high energy isotope goes through the body relatively easy and the difference between absorption and non absorption is small. Still yet, other methods use dual isotopes which is a high cost solution since these have to be replaced often.
As well, the detectors commonly include a collimator in front of it. This results in reduced sensitivity because the collimator is designed to give one to one correspondence between the emission and the detector head.
Therefore, there is a need to solve these problems and also a need for a innovative apparatus for measuring non-uniform attenuation caused by the patient""s body.
According to an aspect of the present invention, there is provided an apparatus for measuring the attenuation of radiation caused by a patient""s body lying in the field(s) of view of a scintillation camera. The apparatus comprises:
(a) a gantry having support, the support defining a space where the patient""s body, in use, is located along a longitudinal axis defined by the space, the gantry supporting a scintillation detector head which comprises a first scintillation detector and a second scintillation detector fixed relative to each other in the form of a xe2x80x9cVxe2x80x9d shape;
(b) a radiation source, disposed at the apex defined by the xe2x80x9cVxe2x80x9d shape, for emitting radiation, the radiation being incident on the patient""s body during use;
(c) a dedicated radiation detector, mounted on the support in opposed relation to the radiation source, for detecting the radiation transmitted through the patient""s body;
(d) the radiation source being adapted to emit the radiation in the form of a beam which sweeps through a selected angle, and the radiation detector being adapted to move in synchronized with the sweeping motion of the beam whereby to detect the attenuation of radiation caused by the patient""s body.
The radiation source comprises a radiation emitter, and an elongated casing for housing the radiation emitter. The elongated casing has a slit formed along the longitudinal axis of the casing such that the radiation can be emitted through the slit from the emitter in the form of a sheet-like beam. The radiation source includes means for rotating the elongated casing such that the radiation emitted through the slit can scan part of the patient""s body in the transversal direction thereof. The radiation emitter includes an isotope emitting a radioactivity, preferably Americium 241.
The radiation detector has an elongated shape whose longitudinal axis is in parallel with the slit of the casing. The radiation detector comprises a collimator, a scintillator for converting the radiation into a light, and a photodetector for sensing the light and measuring the intensity thereof. The photodetector includes a plurality of photomultiplier tubes or a plurality of photodiodes.
The radiation detector is provided with a casing for housing the radiation detector, a track on which the radiation detector moves, and means for driving the radiation detector along the track in line with the scanning movement of the radiation.
In operation, the radiation source and radiation detector are rotated by 180 degrees around the patient""s body, simultaneously while the radiation source scans the patient""s body in the transversal direction thereof, such that the radiation attenuation in all directions around the patient""s body can be measured.
The radiation source includes an x-ray source. The x-ray source and the radiation detector can be utilized to image the patients body while measuring the attenuation of radiation caused by the patient""s body.
According to another aspect of the present invention, there is provided an apparatus for measuring the attenuation of radiation caused by a patient""s body in a scintillation camera. The apparatus comprises:
(a) a gantry having an annular support ring, the annular support ring defining at the centre thereof a cylindrical space where the patient""s body is located along the longitudinal axis of the cylindrical space;
(b) a radiation source, mounted on the annular support ring, for emitting a radiation, the radiation being incident on the patient""s body; and
(c) a dedicated radiation detector, mounted on the annular support ring in opposed relation to the radiation source, for detecting the radiation transmitted through the patient""s body so that the attenuation caused by the patient""s body is measured;
(d) the annular support ring being rotatable around the patient""s body when in use.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.